1. Field of the Invention
This invention relates to starters, and more particularly to an improvement of springs used in the starters.
2. Prior art
A conventional starter for staring a vehicle engine is designed as shown in FIG. 1.
More specifically, the conventional starter device 1, as shown in FIG. 1, comprises: a DC motor 2; an over-running clutch 4 slidably mounted on the output rotary shaft 3 of the motor; an electromagnetic switch 6 provided beside the DC motor 2; and a shift lever 10 having one end engaged with a hook 8 coupled to the plunger 7 of the electromagnetic switch 6 and the other end engaged with the cylindrical rear end portion 9 of the over-running clutch outer portion 4b of the over-running clutch 4 so as to slide the over-running clutch 4 on the output rotary shaft 3.
The electromagnetic switch 6 for operating the shift lever 10 has a cylindrical outer frame 11 which has a wall 11a at one end. The aforementioned plunger 7 is inserted into the end wall 11a. A stationary iron core 12 is disposed at the other end of the outer frame 11 in such a manner as to confront with the plunger 7. The iron core 12 has an end wall 12a which is fixedly fitted in the rear end portion of the outer frame 11, thus forming a frame together with the outer frame 11. A coil bobbin 13 is accommodated in the frame thus formed. An exciting coil 14 is wound on the coil bobbin 13. A return spring 15 is interposed between the iron core 12 and the plunger 7. The iron core 12 has a central through-hole, into which a rod 16 is slidably inserted in such a manner that its one end portion (or front end portion) is extended from the iron core 12 towards the plunger 7. The other end portion (or rear end portion) of the rod 16 supports a movable contact 17.
The starter further comprises: a return spring 18 for returning the rod 16 to a predetermined position; a cap 19 made of resin; and a terminal bolt 20 embedded in the cap 19 so that its inner end serves as a stationary contact 20a with which the movable contact 17 is brought into contact.
The plunger 7 is inserted into the central opening formed in the end wall 11a of the outer frame 11, and it is moved along the central axis of the coil bobbin 13 towards the iron core 12. The plunger 7 has a recess 7a which is opened axially outward. The above-described hook 8 is in the form of a piston and has a flange 8a at the rear end. The hook 8 is slidably inserted into the recess 7a of the plunger 7 and is extended outside passing through the central hole that is formed in a holder 21, which closes the open end of the recess 7a of the plunger 7. The outer end portion of the hook 8 is engaged with the upper end of the shift lever 10. Inside the recess 7a of the plunger 7, a cylindrically coiled spring, namely, a compression spring 22 is interposed between the holder 21 and the flange 8a of the hook 8.
The operation of the conventional starter thus constructed will be described in brief.
When the key switch of the vehicle is turned on, the exciting coil 14 of the electromagnetic switch 6 is energized so that the plunger 7 is moved towards the iron core 12. As a result, the shift lever 10 is turned, whereby the over-running clutch 4 and the pinion 5 integral with the clutch inner portion 4a are slid on the output rotary shaft 3. In this operation, when the pinion 5 abuts against the side of the engine ring gear, the turning of the shift lever 10 is stopped, while the plunger 7 continues to move towards the iron core, the compression spring 22 is compressed, so that the pinion is pushed against the engine ring gear through the shift lever 10.
As the plunger 7 pushes the rod 16, the movable contact 17 is brought into contact with the stationary contact 20a, so that the DC motor 2 is energized. As a result, as soon as the pinion 5 is rotated, it is engaged with the engine ring gear by the elastic force of the compression spring 22.
The relationships between the attracting force of the plunger 7 and the elastic force of the compression spring 22 in the electromagnetic switch is as shown in the characteristic diagram of FIG. 2, in which the vertical axis represents force, and the horizontal axis represents the distance (or gap) q between the plunger 7 and the iron core 12. That is, in FIG. 2, the curves P and P' indicate plunger attracting force characteristics, and the straight line S represents the spring characteristic of the compression spring 22. In general, the power source for the starter is a 12 V storage battery and, in this case, the plunger attracting force characteristic is as indicated by the curve P. However, in practice, the exciting coil 14 is energized with a voltage which is about two-thirds (2/3) of the system voltage because of various factors such as temperature rise; that is, when a voltage of about 8 V is applied to the exciting coil 14, the plunger attracting force characteristic is as indicated by the curve P'.
On the other hand, when the movement of the plunger 7 causes pinion 5 to abut against the engine ring gear, the compression spring 22 becomes effective, and the spring characteristic changes linearly with the movement of the plunger 7.
As shown in FIG. 2, the plunger attracting force characteristic curve P' comes in contact with the characteristic curve S of the compression spring 22 when the attracting force is reduced slightly because of a decrease in the voltage. At the contact point, the plunger attracting force is in balance with the elastic force of the compression spring 22. Accordingly, the plunger 7 is no longer moved towards the iron core 12, as a result of which the DC motor 2 is not energized. This difficulty may be overcome by using a compression spring smaller in elastic force. However, the method is not practical, because the elastic force thereof may not be large enough to cause the pinion to engage with the engine ring gear.
In the electromagnetic switch thus constructed, the coiled spring 15 for returning the plunger 7 is disposed between the plunger 7 and the stationary iron core 12 in such a manner that it is positioned closest to the inner cylindrical wall of the coil bobbin 13 as shown in FIG. 1, thus reducing the magnetic cross section of the magnetic path. Accordingly, the force of attracting is decreased, and especially the initial force of attracting the plunger is decreased.
In order to overcome this difficulty, the conventional magnetic switch 6 has been improved as shown in FIG. 3. That is, in the improved magnetic switch, the coiled spring 15 is disposed between the plunger 7 and the stationary iron core 12 in such a manner that it is located closest to the central axis of the coil bobbin 13 so as to maintain the magnetic cross section of the magnetic path.
However, the electromagnetic switch is still disadvantageous in the following point: In the electromagnetic switch, the coiled spring 15 is inserted into a recess which is formed along the through-hole of the iron core 12, into which the rod 16 is inserted, in such a manner that it is deep enough to support the base portion of the spring 15. Therefore, the dimension a of the rod supporting portion of the iron core is decreased, and the rod 16 will have less support, thereby resulting in increased perpendicular movement of the rod 16 relative to the spring 15.